[Client] Transient User Alts investigation tab (#7511)

* [Client] Transient User Alts investigation tab

New TabAlts tab with user search, depth selector (1-3), and an alt table showing per-row match evidence (Via, Depth, Shared fan-out) alongside the account data. Sends Command_GetUserAlts with the requested depth and degrades gracefully against old servers (missing evidence fields render as dashes). Wired as a transient single-reuse tab via TabSupervisor::openTabAlts with no menu action or persisted setting; closed on disconnect like the other staff tabs.

* Fix remaining review threads on the user alts tab

- Switch to the User Alts tab when it is created, so the first investigate
  click does not open it in the background.
- Treat an empty RespOk as "user not found" and use a generic message for
  error codes, matching what the server actually returns.
- Exclude the depth-0 seed row from the related-account count.
- Guard the reply with the requested user name so a stale response cannot
  overwrite a newer one.
- Move the duplicated formatEpoch body into widgets/utility/epoch_format.h.

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

cockatrice/src/interface/widgets/utility/epoch_format.h

@@ -0,0 +1,27 @@
+/**
+ * @file epoch_format.h
+ * @brief Shared timestamp formatting for the moderation tables.
+ */
+
+#ifndef EPOCH_FORMAT_H
+#define EPOCH_FORMAT_H
+
+#include <QDateTime>
+#include <QString>
+
+/**
+ * @brief Format a unix timestamp the way the moderation tabs show dates.
+ *
+ * @param ts          Seconds since the epoch; 0 means "unknown".
+ * @param unknownText Label returned for 0, translated by the caller so the
+ *                    translation context stays with the tab.
+ */
+inline QString formatEpochTimestamp(quint64 ts, const QString &unknownText)
+{
+    if (ts == 0) {
+        return unknownText;
+    }
+    return QDateTime::fromSecsSinceEpoch(ts).toLocalTime().toString(QStringLiteral("yyyy-MM-dd HH:mm"));
+}
+
+#endif // EPOCH_FORMAT_H